Because FUCK YOU. That's why.

Car companies want you to go back to the dealership to have codes read. They had to be sued into releasing the code's meanings so independent shops and owners could do their own repairs.

Even though most car owners are knuckle draggers who don't understand oil changes or air filter changes, they don't want to make it any easier to do repairs outside of dealerships. I know someone who swears by dealerships and was very confused when I asked if he ever changed the air filter to improve fuel economy. For some reason, he thought his car didn't have one.

There is a standard connector which existed before big screens landed in cars, the OBD2 connector. Dongles are cheap and you can read the output from your phone or computer. Some dongles support bluetooth. The connector is mandated in some markets and I guess that makes it less interesting to add a redundant interface inside of the car. It's fun to try if you're interested. Manufacturers can extend the error codes IIRC.

Tesla has a service mode on the display through which you can scan the car for faults, run a battery test, ... It is password protected but the password is publicly available.

The sad answer is that those displays exist to cut costs, not to make your ownership experience better.

Also the dealers want you to come to them for that CEL, and the companies don’t want to piss off the dealers.

Who is going to look at that stuff apart from technicians? Most users have no clue how the functionality of their vehicle is achieved and they don't care.

For argument's sake, let's assume there is a userbase for this type of information. It would be possible to show diagnostic information like DTC or run DID routines from the dashboard but this is already possible from any cheap offboard tester, via a phone app or laptop.

The reality is that even if an OEM wanted to provide detailed diagnostic information, they don't know it either because the information isn't disclosed by their supply chain. Companies such as Bosch, who supply brake ECU, are extremely tight lipped about their intellectual property. When something goes wrong we use a special development version of the ECU to record the associated software variables during the fault and present that as evidence but we don't have access to the source code.

Modern products are not designed to be repaired. They want us to continually buy new shit. Basically anything with software in it is an absolute nightmare to maintain. It makes me depressed just thinking about what a clusterfuck this landscape is.

Source: control system engineer for a large OEM.

They want you to go to the dealership...

The only reason you can read the codes at all is because of regulation... For now thanks Trump

Why don't all computers have everything digital ever made? https://en.wikipedia.org/wiki/Artificial_scarcity

If they wanted you to have that info, the software that powers the code reader would already be in the cars computer and would show human readable errors.

They don't

As someone pointed out on the comment, one of the reason is Money, the other one is safety and security ~~based on Vehicle manufacturer claim~~.

Yet, nobody can stop you to sniff and monitor your own car CAN/OBD2 interface using as simple as cheap microcontroller and compatible OBD2 interface kit (e.g CAN Transceiver/Controller kit).

EDIT: So far this curated list helpful to me for CAN bus based protocol.

The same reason they won't let you buy the dealership scan software for under 10k. Almost every maker has an in house scanner and due to standards they only need to provide certain data to non dealer level tools and I believe the standard only exists for gas powered vehicles that need to provide OBD2 data. Plenty of makers (BMW is horrible about this) stuff away data where a normal obd scanner just won't access and it's gotten much worse with the overuse of CANBus (I sure love when my trunk lid sensor prevents my fucking car from starting).

Thats where your snapon and other third party scanners start bringing a gap, but even those are extremely pricey and need to be updated constantly and even those usually won't do EVERYTHING.

Fwiw the cheapest and best way I've found is basically to pirate the dealer software and get a compatible knockoff scanner (vxdiag for example). I have Ford IDS and a couple others this way but assume that the software is gonna install something malicious and dedicate an old Thinkpad or something to it.

Depending on the age of your vehicle something like Torque Pro is extremely useful. I have mine monitoring transmission temp, long and short term fuel trims, O2 sensor signals, voltage, mass air speed, intake temp. It's more than enough data to see something coming long before it becomes an issue.

Surprisingly, Teslas actually do. You have to access a hidden menu, but it's well documented and easy to do.